In an effort to increase the information carrying capacity of band-limited channels, techniques have been developed which permit analog, e.g., voice, and digital data signals to be simultaneously communicated in a single channel. One such arrangement to accomplish this, known as statistical multiplexing, uses the silence portions of an analog speech signal to transmit data. There are several shortcomings associated with statistical multiplexing. For one, silence detection can be a problem in environments, such as mobile communications systems, with significant background noise. For another, the quality of the analog speech signal may be degraded due to clipping at the beginning and end of this signal. For still another, both the data throughput and the delay experienced by the transmitted data are variable. Time division and frequency division multiplexing are other techniques which have been used to provide simultaneous voice and data communications capabilities for a single channel. With frequency-sharing, the division of the available bandwidth in many applications results in low data rates and/or lower speech quality. With time-sharing techniques, data rates of 9.6 to 19.2 kilobits/second have been transmitted along with digitized speech. By varying the voice/data allocations, a variety of results ranging from low-quality speech and high-data rates to high-quality speech and low-data rates are possible.
While implementation of time division multiplexing of voice and data signals can utilize standard speech coders and modems, the cost of implementation may exceed the desired cost objectives of certain applications.
More recently, in a patent application U.S. Ser. No. 08/076,505 entitled "Simultaneous Analog And Digital Communication", filed Jun. 14, 1993, issued as U.S. Pat. No. 5,448,555 on Sep. 5, 1996, assigned to the present assignee and incorporated herein by reference, a technique is disclosed wherein a voice signal is superimposed upon quadrature analog carrier signals whose amplitudes have been modulated with digital data. Further disclosed in this application is the use of linear prediction in both the transmitter and receiver to improve the recovery of the voice and data signals. The use of linear prediction in the transmitter requires that the prediction coefficients be transmitted to the receiver. Such coefficient transmission reduces the available bandwidth. While the bandwidth required for coefficient transmission can be reduced through the utilization of lower-order linear prediction schemes, such schemes reduce the voice and data signal recovery benefits obtainable for linear prediction. As a result, the user is faced with the quandary of either maximizing the data and voice recovery process and reducing the bandwidth usable for voice and data signal transmission or maximizing this bandwidth while reducing the accuracy of the voice and data signal recovery process. Since the transmission of a voice and data signal, one signal superimposed upon the other, will likely find widespread use, it would be desirable if a technique could be developed for such applications which would provide the signal recovery benefits of linear prediction but does not require the communication of prediction coefficients from the transmitter to the receiver.